U.S. patent number 6,402,579 [Application Number 09/254,302] was granted by the patent office on 2002-06-11 for electrode deposition for organic light-emitting devices.
This patent grant is currently assigned to Cambridge Display Technology Limited. Invention is credited to Peter Devine, Karl Pichler.
United States Patent |
6,402,579 |
Pichler , et al. |
June 11, 2002 |
**Please see images for:
( Certificate of Correction ) ** |
Electrode deposition for organic light-emitting devices
Abstract
There is disclosed an organic light-emitting device having at
least one layer of light-emissive organic material arranged between
first and second electrodes, one of the first and second electrodes
being a multilayer structure, each layer of the multilayer
structure being a DC magnetron sputtered layer. There is also
disclosed an organic light-emitting device having two or more
layers of light-emissive organic material arranged between first
and second electrodes, an uppermost layer of the organic material
being more resistant to sputter deposition than an underlying layer
of the organic material, and the electrode formed over the
uppermost layer of organic material being a sputtered layer. There
are also disclosed methods for making such structures.
Inventors: |
Pichler; Karl (Hopewell
Junction, NY), Devine; Peter (Milton, GB) |
Assignee: |
Cambridge Display Technology
Limited (GB)
|
Family
ID: |
27451518 |
Appl.
No.: |
09/254,302 |
Filed: |
December 23, 1999 |
PCT
Filed: |
September 04, 1997 |
PCT No.: |
PCT/GB97/02395 |
371(c)(1),(2),(4) Date: |
December 23, 1999 |
PCT
Pub. No.: |
WO98/10473 |
PCT
Pub. Date: |
March 12, 1998 |
Foreign Application Priority Data
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|
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Sep 4, 1996 [GB] |
|
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9618473 |
Sep 4, 1996 [GB] |
|
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9618474 |
Sep 4, 1996 [GB] |
|
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9618475 |
Jun 12, 1997 [GB] |
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9712295 |
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Current U.S.
Class: |
445/24 |
Current CPC
Class: |
H01L
51/5203 (20130101); H05B 33/10 (20130101); H05B
33/12 (20130101); H05B 33/26 (20130101); H01L
51/5215 (20130101); H01L 51/5231 (20130101) |
Current International
Class: |
H01L
51/50 (20060101); H01L 51/52 (20060101); H05B
33/26 (20060101); H05B 33/10 (20060101); H05B
33/12 (20060101); H05B 033/10 () |
Field of
Search: |
;445/24 |
References Cited
[Referenced By]
U.S. Patent Documents
Foreign Patent Documents
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0278757 |
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Aug 1988 |
|
EP |
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0443861 |
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Aug 1991 |
|
EP |
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0468437 |
|
Jan 1992 |
|
EP |
|
0468438 |
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Jan 1992 |
|
EP |
|
0468439 |
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Jan 1992 |
|
EP |
|
0549345 |
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Jun 1993 |
|
EP |
|
0553950 |
|
Aug 1993 |
|
EP |
|
0563009 |
|
Sep 1993 |
|
EP |
|
0605739 |
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Jul 1994 |
|
EP |
|
0684753 |
|
Nov 1995 |
|
EP |
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7-85973 |
|
Mar 1995 |
|
JP |
|
7-150137 |
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Jun 1995 |
|
JP |
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8-264278 |
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Oct 1996 |
|
JP |
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9-35871 |
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Feb 1997 |
|
JP |
|
WO 94/03031 |
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Feb 1994 |
|
WO |
|
WO 95/24056 |
|
Sep 1995 |
|
WO |
|
Other References
Nguyen et al., "Influence of Thermal And Plasma Treatments On The
Electrical Properties Of Poly (para-phenylene vinylene)-Based
Diodes," Synthetic Metals, 72, pp. 35-39, 1995. .
Bulovic et al., "A Surface-Emitting Vacuum-Deposited Organic
Light-Emitting Device," Applied Physics Letters, Jun. 2, 1997, vol.
70, No. 22, pp. 2954-2956, XP002051289. .
Bulovic et al., "Transparent Organic Light-Emitting Devices,"
Applied Physics Letters, May 6, 1996, vol. 68, No. 19, pp.
2606-2608. .
Suzuki et al., "Organic Light-Emitting Diodes With Radio Frequency
Sputter-Deposited Electron Injecting Electrodes," Applied Physics
Letters, Apr. 15, 1996, vol. 68, No. 16, pp. 2276-2278,
XP000585170..
|
Primary Examiner: Ramsey; Kenneth J.
Claims
What is claimed is:
1. A method of fabricating an organic light-emitting device,
comprising the steps of:
forming a first electrode over a substrate;
forming two or more layers of light-emissive material over the
first electrode, wherein the uppermost layer of organic material is
more resistant to sputter deposition than the underlying layer of
organic material; and
forming a second electrode over the uppermost layer of organic
material, wherein the second electrode comprises at least one
layer, the layer adjacent the uppermost layer of organic material
being a sputtered layer.
2. A method of fabricating an organic light-emitting device
according to claim 1, wherein the uppermost layer of organic
material is a conjugated polymer.
3. A method of fabricating an organic light-emitting device
according to claim 1, wherein the step of forming the multilayered
electrode structure comprises the steps of sputtering a first layer
of conductive material having a low work function over the at least
one layer of organic material and sputtering a second layer of
conductive material over the first layer of conductive material,
the first layer of conductive material being substantially thinner
than the second layer of conductive material.
4. A method of fabricating an organic light-emitting device
according to claim 1, wherein the first layer of conductive
material has a thickness of at most 20 nm.
5. A method of fabricating an organic light-emitting device
according to claim 1, wherein the step of forming the multilayered
electrode structure comprises the steps of sputtering a first layer
of conductive material having a high work function over the at
least one layer of organic material and sputtering a second layer
of conductive material over the first layer of conductive material,
the first layer of conductive material being substantially thinner
than the second layer of conductive material.
6. A method of fabricating an organic light-emitting device
according to claim 5, wherein the first layer of conductive
material has a thickness of less than 10 nm.
Description
The field of the invention relates to organic light-emitting
devices with improved electrodes and their deposition.
Organic light-emitting devices (OLEDs) such as described in U.S.
Pat. No. 5,247,190 assigned to Cambridge Display Technology Limited
or in Van Slyke et al, U.S. Pat. No. 4,539,507, the contents of
which are herein incorporated by reference and example, have great
potential for use in various display applications, either as
monochrome or multi-colour displays. Principally, an OLED consists
of an anode that injects positive charge carriers, a cathode that
injects negative charge carriers and at least one organic
electroluminescent layer sandwiched between the two electrodes.
Typically although not necessarily, the anode would be a thin film
of, for example, indium-tin-oxide (ITO), a semi-transparent
conductive oxide which is commercially readily available already
deposited on glass or plastic substrates. The organic layer(s)
would then normally be deposited onto the ITO-coated substrate by,
for example, evaporation/sublimation or spin-coating,
blade-coating, dip-coating or meniscus-coating. The final step of
depositing the cathode layer onto the top organic layer is normally
performed by thermal evaporation or sputtering of a suitable
cathode metal in vacuum. Layers of Al, Ca or alloys of Mg:Ag or
Mg:In, or Al alloys are often used as cathode materials.
One of the key advantages of the OLED technology is that devices
can be operated at low drive voltages, provided that suitable
electroluminescent organic layers and electrodes with good
efficiencies for the injection of positive and negative charge are
used. In order to achieve good performance in OLEDs it is of great
importance to optimise all the individual layers, the anode, the
cathode and the organic layer(s), as well as the interfaces between
the layers.
A cathode of high quality is of great importance to achieve overall
high performance in OLEDs, judged on criteria such as power
efficiency, low drive voltage, shelf life, operating life, and
stability in stringent environmental conditions such as high
temperature and/or high humidity, etc. The criteria for the quality
of the cathode are in particular but not exclusively the work
function, corrosion resistance, morphology and barrier properties,
adhesion to the polymer and sheet resistance.
Metallic cathode layers for OLEDs are most commonly deposited by
simple thermal evaporation of the cathode material in vacuum.
Similarly, cathode layers consisting of a metal alloy can be
deposited by thermal evaporation from two or more sources
containing the alloy constituents and by choosing appropriate
relative depositing rates to achieve the desired relative alloy
composition.
However, simple thermal evaporation of metals onto OLEDs to form a
cathode layer can result in poor adhesion between the cathode and
the top organic layer and, very often, the morphology of the
evaporated layer is polycrystalline with large average grain size
such that there is a high density of grain-boundary for diffusion
of ambient gases such as oxygen and moisture into the device. Poor
adhesion and large grain-size polycrystalline morphology can
severely deteriorate the OLED performance, in particular
environmental stability (device shelf-life and operating life,
corrosion of the cathode).
The same issues (adhesion, morphology) apply to the case in which
an OLED is built up from the cathode, i.e. when the cathode is
deposited on the substrate with the subsequent deposition of the
organic layer(s) and as the final step deposition of the anode on
top of the top organic layer.
Simple thermal evaporation from different sources of elemental
metal or evaporation of a ready-made alloy to obtain cathodes in
alloy form also has problems. For example, if a cathode alloy layer
comprising reactive low work function elements, such as alkali or
earth-alkali metals, is required the processing and/or handling of
these elements in a normal environment in air may be difficult if
not impossible. Alternatively, if an alloy itself is evaporated,
the alloy composition of the deposit (cathode) may be difficult to
control due to, for example, different thermal properties and
differential evaporation rates of the source-alloy
constituents.
It is thus an object of the present invention to provide a
structure and method of fabrication for an organic
electroluminescent device that overcomes, or at least minimises,
the above-described problems.
According to a first aspect of the present invention there is
provided an organic light-emitting device, comprising at least one
layer of a light-emissive organic material arranged between first
and second electrodes, wherein at least one of the electrodes is a
multi-layered structure, each layer of the multi-layered structure
being a DC magnetron sputtered layer.
This first aspect of the invention also provides a method of
fabricating an organic light-emitting device, comprising the steps
of:
forming an electrode over a substrate;
forming at least one layer of a light emissive organic material
over the electrode; and
forming a multi-layered electrode structure over the at least one
layer of organic material, each layer of the multi-layered
structure being a DC magnetron sputtered layer.
This first aspect of the invention further provides a method of
fabricating an organic light-emitting device, comprising the steps
of:
forming a multi-layered electrode structure over a substrate, each
layer of the multilayered structure being a DC magnetron sputtered
layer;
forming at least one layer of a light-emissive organic material
over the multi-layer ed electrode ; and
forming an electrode over the at least one layer of organic
material.
According to a second aspect of the present invention there is
provided an organic light-emitting device, comprising:
a first electrode;
two or more layers of light-emissive organic material formed over
the first electrode, wherein the uppermost layer of organic
material is more resistant to sputter deposition than the
underlying layer of organic material; and
a second electrode formed over the uppermost layer of organic
material, wherein the second electrode comprises at least one
layer, the layer adjacent the uppermost layer of organic material
being a sputtered layer.
This second aspect of the invention also provides n organic
light-emitting device, wherein-the uppermost layer of organic
material is substantially resistant to sputter deposition.
Thus, the first and second aspects of the invention provide an
organic electroluminescent device with a metallic cathode of
compact morphology with low average grain size and good adhesion to
the adjacent layer of the OLED stack with the said cathode laid
down by sputter deposition. Good adhesion between the cathode and
the adjacent layer minimises delamination and the ingress of, for
example, oxygen, moisture, solvents or other low molecular weight
compounds at/along said interface. Also, the compact morphology of
the cathode metal layer minimises diffusion of ambient species such
as oxygen, moisture, solvents or other low molecular weight
compounds into the OLED through the cathode layer itself. Said
cathode forms the electron injecting electrode for an OLED with at
least one electroluminescent organic layer between said cathode and
an anode that injects positive charge carriers. The organic
electroluminescent layer(s) are preferably but not necessarily
conjugated polymers.
In the first aspect of the invention the sputter deposition process
is DC magnetron sputtering. In the second aspect of the invention
the light-emissive organic material comprises two or more layers,
the uppermost layer being more sputter resistant to sputter
deposition than the underlying layer of organic material.
FIG. 1 illustrates an OLED;
FIG. 2 illustrates an OLED in accordance with a first embodiment of
the present invention;
FIG. 3 illustrates an OLED in accordance with a second embodiment
of the present invention;
FIG. 4 illustrates a further OLED; and
FIG. 5 illustrates an OLED with an improved anode.
In an illustrative example, OLED is formed by a semi-transparent
anode deposited onto a transparent supportive substrate. The
substrate is, for example, a sheet of glass of thickness between 30
.mu.m and 5 mm but preferably .ltoreq.1.1 mm or alternatively a
sheet of plastic such as polyester, polycarbonate, polyimide,
poly-ether-imide or the like. The semitransparent anode is
preferably but not necessarily a thin layer of conductive oxide
such as indium-tin-oxide, doped tin-oxide or zinc-oxide. The
organic layer(s) deposited on top of the anode/substrate is/are
preferably but not necessarily one or more layers of an
electroluminescent conjugated polymer such as described in U.S.
Pat. No. 5,247,190, typically of the order of 100 nm thick.
Alternatively the organic layer(s) could be low molecular weight
compounds such as described in the U.S. Pat. No. 4,539,507 or
indeed a combination of a conjugated polymer with a low molecular
weight compound. The cathode, deposited by means of sputtering on
top of the top organic layer, is a metal capable of injecting
electrons into the adjacent organic layer and is typically but not
necessarily of the order of 100-500 nm thick. Examples of such
metals are--but are not limited to--Al, Mg, In, Pb, Sm, Tb, Yb or
Zr. The adhesion and compact morphology of such sputter deposited
metal layers is far superior compared to those deposited by thermal
evaporation; hence device stability and corrosion resistance of the
cathode is significantly improved, particularly in cases in which
ambient oxygen, moisture, solvents or other low molecular weight
compounds are present during the post-cathode deposition
fabrication process or the operation of the devices.
Alternatively, said cathode deposited by sputtering is an alloy
sputtered from an alloy sputtering target. Such alloys are--but not
limited to--alloys of Al, Zr, Mg, Si, Sb, Sn, Zn, Mn, Ti, Cu, Co,
W, Pb, In or Ag or combinations thereof and containing low work
function elements such as Li, Ba, Ca, Ce, Cs, Eu, Rb, K, Sm, Na,
Sm, Sr, Tb or Yb. A typical such alloy would, for instance, be a
commercially available Al95%/Li2.5%/Cu1.5/Mg1% alloy. Said low work
function elements are in pure elemental form normally very
susceptible to oxidation and corrosion by moisture such that
handling in a normal laboratory environment is very problematic if
not impossible. However, in a matrix of more stable metals such as
listed above these low work function elements can--at sufficiently
low concentrations in the matrix element--be stable enough to allow
sputter target fabrication, handling of the target and sputter
deposition of the target alloy onto an OLED in standard sputter
equipment in a normal laboratory or manufacturing environment with
subsequent stability of the cathode on the OLED after sputter
deposition. In this example the low work function element(s) act to
improve electron injection and device efficiency, the matrix
element provides environmental stability and the sputter deposition
process again improves adhesion, gives a compact morphology and--by
choice of the target composition and the sputter conditions--allows
to control the composition of the cathode alloy as deposited. The
alloy morphology achieved specifically by DC magnetron sputter
deposition can act to minimise, for example, segregation and
diffusion effects within the cathode alloy after the deposition. In
this way OLEDs can be produced in ambient conditions with low work
function cathodes which provide efficient electron injection into
the organic layer(s).
Turning now to FIG. 1, a first OLED is realised by depositing a ca.
1000 .ANG. thick layer 3 of poly(p-phenylene vinylene) (PPV) on a
substrate 1 pre-coated with a layer 2 of semi-transparent
indium-tin-oxide (ITO) having a transparency of at least 80% in the
visible range and a sheet resistance of less than 100 ohms/square,
such as described in U.S. Pat. No. 5,247,190. A cathode, comprising
a ca. 150 nm thick layer 4, is deposited on top of the PPV layer 3
by DC magnetron sputtering under the following conditions:
Al95%/Li2.5%/Cu1.5%/Mg1% target, sputtering in constant power mode
at ca. 3.2 W/cm.sup.2 (100 mm diameter target and 250 W), pressure
ca. 5.times.10.sup.-3 mbar at 25 sccm argon flow (base pressure ca.
1.times.10.sup.-6 mbar), target voltage 400-410V, target-substrate
distance 75 mm and a deposition rate of ca. 1 nm/sec. An OLED made
this way has a low turn-on and operating voltage (<5V), improved
efficiency over similar devices with cathodes of even pure Ca, and
the cathode material in the form of a sputter target and as a film
deposited on the OLED is environmentally stable and can be handled
in a normal laboratory environment.
In another example the OLED deposited by sputtering is a double
layer structure in which the first layer of the cathode, adjacent
to the top organic layer, is a thin layer of a low work function
element or an alloy containing a low work function element, such as
described herein above with respect to the cathode, and in which
said thin first low work function element containing layer is
capped with a second thick layer of a stable conductive layer. The
thin first cathode layer can, for instance, be an alloy containing
Li and is typically .ltoreq.20 nm but preferably .ltoreq.5 nm
thick. The thick second cathode layer can, for instance, be Al,
Al-Cu, Al-Si or other alloys, is preferably at least 100 nm thick
and acts to protect the thin first cathode layer, provides
environmental stability to the device and provides a low sheet
resistance of the cathode. Alloys do very often show surface
segregation effects whereby, for example, one of the alloy elements
gets preferentially enriched at interfaces/surfaces. A double
cathode layer structure in which low work function elements are
only contained to a small percentage and that only in a thin first
cathode layer minimises problems with the enrichment of, for
instance, the low work function element at the interface with the
adjacent organic layer which could, for example, lead to increased
reactivity or intolerable amounts of diffusion of the low work
function element into the organic layer(s) which could result in
intolerable levels of subsequent doping of the organic layer and a
possible reduction in device efficiency. Said thin first cathode
layer can readily be obtained and its thickness be controlled by,
for example controlling the pass-by speed of the sample over the
sputter zone, together with the deposition rate.
Preferably, in any such two layer thin/thick cathode, the ratio of
thickness of the first layer of conductive material to the second
layer of conductive material is at least 20:1.
In another example and referring to the two layer thin/thick
cathode structure described above, said second, thick conductive
cathode layer could alternatively be a semi-transparent conductive
oxide, such as for instance indium-tin-oxide (ITO).
In another example, referring to the two layer thin/thick cathode
structure described above, only the second thick layer of the
cathode is deposited by sputtering and the first thin layer is
deposited by other means such a thermal evaporation. This is useful
in cases in which, for instance, the top organic layer is sensitive
to damage by the sputter process--without or with employing
reactive sputtering such as used in the reactive sputter deposition
of semi-transparent conductive oxides. In this embodiment the first
layer would still form the electronic interface of the cathode with
the adjacent organic layer but the second thick DC magnetron
sputtered layer would form a morphologically compact protective and
conductive film.
In another example a sputtered cathode deposited as a top layer of
the OLED also provides, due to its good adhesion, compact
morphology and good barrier properties, as an encapsulation layer
for the device which provides protection against the ingress into
the device of undesirable substances such as reactive gases and
liquids which may be present during subsequent processing steps
and/or the shelf life and operating life of the device.
According to another example there is provided an OLED in which
improved charge carrier balance and device efficiency is achieved
by the introduction of a sputtered very thin film of a
stoichiometric or sub-stoichiometric dielectric under the cathode
with said thin film of dielectric being .ltoreq.5 nm thick. OLEDs
often show better injection and transport for positive charge
carriers compared to negative charge carriers. It has been found
that thin films of dielectrics of thicknesses .ltoreq.5 nm can have
substantially higher transmission rates for electrons than for
holes (positive charge carriers); this can be particularly the case
for stoichiometric and sub-stoichiometric metal oxides. Therefore
such thin dielectric layers between the cathode and the top organic
layer can improve device efficiency.
Thus, according to this example, an OLED is composed of an anode on
a supporting substrate (such as ITO-coated glass) with at least one
organic electroluminescent layer which is covered with a thin
(.ltoreq.5 nm) layer of a dielectric which is then coated with a
metallic cathode according to the first and/or second aspects of
the invention. The dielectric is preferably, but not limited to,
metal oxides, either stoichiometric or sub-stoichiometric (oxygen
deficient), and deposited onto the top organic layer by sputtering
with the other elements of the OLED according to the first and/or
second aspects of the invention. Typically but not necessarily the
oxide would be formed by reactive DC magnetron sputtering in oxygen
with Al, Mg, Zr or other elemental metal targets or indeed an oxide
reactively sputtered from an Al:Mg, Al:Li, Al:Li:Cu:Mg, or other
alloy sputter target.
Referring now to an OLED with a stable low work function cathode
and improved efficiency is realised by depositing a ca. 100 nm
thick layer 3 of poly(p-phenylene vinylene) on a substrate 1
precoated with a layer 2 of semi-transparent indium-tin-oxide
having a transparency of at least 80% in the visible range and a
sheet resistance of less than 100 Ohms/square, such as described in
U.S. Pat. No. 5,247,190. The PPV layer 3 is coated with a ca. 3 nm
thick layer 5 of a stoichiometric oxide of Al95%/Li2.5%/Cu1.5%/Mg1%
realised by reactive DC magnetron sputtering under the following
conditions: target-substrate distance 75 mm, base pressure
1.times.10.sup.-6 mbar, power supply mode at constant voltage of
310V, current density 6.2 mA/cm.sup.2, pressure 5.times.10.sup.-3
mbar, 25 sccm argon flow, 2 sccm oxygen flow, deposition time 7
sec. A cathode, comprising a ca. 150 nm thick layer 4, is deposited
on top of the thin oxide layer 5 by a DC magnetron sputtering in
the manner as described above. The cathode preferably comprises a
conductive layer having a thickness of from 100 to 500 nm. This
device shows improved efficiency over an equivalent device with the
same cathode but without the thin oxide layer.
An OLED with a sputtered cathode, as described above and in which
the organic layer adjacent to the cathode (the top organic layer)
is a conjugated polymer which is particularly amenable to
subsequent sputter deposition and protects underlying organic
layers from the sputtering process but does otherwise not
deteriorate the overall device performance beyond intolerable
levels is described in another example.
With respect to this example it has been found that the nature of
the top organic layer can greatly determine the success of a
subsequent sputter deposition, as judged by the efficiency of
electron injection, the OLED driver voltage or reliability and
yield of device manufacture and indeed damage to the top organic
layer due to the sputter deposition process itself. It has been
found that sputter deposition of metal and alloys can be readily
used on morphologically very compact and mechanically tough
conjugated polymers such as, for example, poly(p-phenylene
vinylene), PPV, without any obvious signs of great damage to the
polymer and the interface.
This is less so that case with "softer" soluble polymers such as,
for example, soluble derivatives of PPV with di-alkoxy side chains.
In the case of these softer polymers the sputter deposition process
can more easily result in a damage of the top polymer layer and the
interface such that, for example, the device drive voltage is
increased and devices are more prone to shorting.
According to the described example, in OLEDs with "softer" polymers
as the main active and electroluminescent layers this/these said
layer(s) are protected with a thin top layer of a morphologically
very compact and mechanically tough conjugated polymer, such as but
not exclusively PPV, in order to make the device more amenable to a
final sputter deposition of a cathode with or without minimised
damage to the device and with the subsequent advantages of a
sputtered cathode as outlined in the hereinabove. Said layer of
protective tough polymer is typically of the order of a few tens of
nm thick but the thickness is preferably in the range of 5 to 20
nm. The optical, electronic and transport properties of, for
example, PPV on top of di-alkoxy derivatives of PPV are such that
device properties such as efficiency or emission colour are not
greatly changed compared to a device without the protective PPV
layer.
In a second specific example illustrated with reference to FIG. 3,
an OLED is realised by depositing a ca. 100 nm thick layer 6 of
poly(2-methoxy,5-(2'-ethyl-hexyloxy)-p-phenylene vinylene)
[MEH-PPV] spun from a xylene solution on a substrate 1 pre-coated
with a layer 2 of semi-transparent indium-tin-oxide. The MEH-PPV
layer 6 is then coated with a thin layer 3 of poly(p-phenylene
vinylene) (ca. 20 nm after conversion). The spinning of the PPV
precursor solution on top of the MEH-PPV layer 6 is possible due to
the use of immiscible solvents for the MEH-PPV and the PPV
precursor, and the conversion of the PPV precursor to PPV does not
apparently harm the MEH-PPV. A cathode, comprising a ca. 150 nm
thick layer 4, is deposited on top of the thin PPV layer 3 by DC
magnetron sputtering in the manner as described above. Such an OLED
gives the orange/red emission characteristic of MEH-PPV and has a
low drive voltage (ca. 5-6 V) due to the air-stable low work
function sputter-deposited cathode alloy. Equivalent devices in
which the cathode is sputtered in the same way but directly on top
of MEH-PPV layer, i.e. without a PPV buffer layer, tend to have
drive voltages in excess of 10 V and are very prone to shorting,
due possibly to damage of the upper surface of the MEH-PPV layer
during the sputtering process.
In another example the cathode is already deposited onto a
supporting substrate of an OLED. The substrate with the cathode is
coated with at least one organic electroluminescent layer and the
second and top electrode is the anode deposited by sputtering.
In one arrangement of this example said top sputtered anode layer
is a layer of a high work function metal such as but not limited to
C, Ag, Au, Co, Ni, Pd, Pt, Re, Se or alloys thereof, or doped
semiconducting compounds or more generally conductive layers with
work functions above 4.7 eV. Alternatively said anode is a layer of
a conductive sputtered oxide, such as but not limited to
indium-tin-oxide, or doped Sn-oxide or Zn-oxide.
In another arrangement of this example, referring to the case in
which said top anode is a conductive oxide deposited by reactive
sputtering, the top organic layer is first coated with a thin layer
of a high work function element or alloy such as but not limited to
C, Au or Pt by either sputter deposition or other means such as
thermal evaporation, before the thick sputter deposited conductive
oxide is applied. In this case damage to the top organic layer by
the reactive sputter deposition process is minimised. In the cases
in which the anode needs to be at least semi-transparent the thin
interface layer between the top organic layer and the
semi-transparent oxide needs to be less than 10 nm but preferably
less than 5 nm thick in order to preserve transparency.
A further OLED, referring to FIG. 4, is realised by coating a ca.
100 nm thick layer 3 of poly(p-phenylene vinylene) on top of a
substrate 1 coated with a layer 4 of Al which acts as the cathode.
After conversion of the PPV precursor polymer to PPV the PPV is
coated with a thin (<5 nm) layer 7 of thermally evaporated Au
which is then capped with a 150 nm thick layer 2 of
indium-tin-oxide in a standard commercial reactive DC magnetron ITO
deposition process. The thin layer 7 of Au ensures the efficient
injection of positive charge carriers but also provides a buffer
layer which protects the underlying PPV layer 3 during the reactive
ITO deposition process.
The present invention thus describes organic electroluminescent
device structures and methods of fabricating the same in which the
second, i.e. top, electrode is at least in part realised by
sputtering. Said sputtered electrode layer assures a compact dense
electrode morphology with good barrier and adhesion properties and
the sputter deposition process itself also allows the use of alloys
which combine environmental stability with desired electronic
properties such as low work function.
A protective insulating layer may be sputtered over the sputtered
top electrode without exposing the OLED to ambient atmosphere
between the deposition of the top electrode and said insulating
layer and said insulating layer being, for example, an oxide or a
nitride.
A method of fabrication of an organic electroluminescent device
wherein at least one of the electrodes is at least in part realised
by sputter deposition of a metallic element or alloy or conductive
oxide or semiconductor is also disclosed.
It has been found that one of the mechanisms which limits device
performance and in particular device lifetime can be the
degradation of the organic layer adjacent to the inorganic oxide
anode and the interface between said anode and the adjacent organic
layer due to, for example, oxygen released from said oxide anode,
with the organic layer. In this context it has been found that the
incorporation of thin layers of, for example, semiconductive
polymers such as polyaniline, as a first organic layer on top of
the inorganic oxide anode results in improved device
characteristics and operating stability. The introduction of such
additional layers as first organic layer on top of the inorganic
oxide anode can, however, introduce other problems such as
deteriorated adhesion, mixing of said first organic layer with
subsequent layers which may in some cases be undesired, a
deterioration in the wetting and coating properties of subsequent
layers, problems with uniform deposition of thin layers of said
first organic layer or problems with the stability of said first
organic layer under device operation.
FIG. 5 illustrates a structure for an organic light-emitting device
that has the advantage of separating the inorganic oxide
semi-transparent anode from the first organic layer, but maintains
effective injection of positive charge carriers and avoids some of
the above mentioned problems.
The structure comprises at least one layer of a light-emissive
organic material arranged between an anode and a cathode for the
device, wherein the anode comprises a first light-transmissive
layer of an inorganic oxide and a second light-transmissive layer
of a conductive material having a high work function arranged
between the at least one layer of organic material and the first
layer of inorganic oxide, the second layer of conductive material
being substantially thinner than the first layer of inorganic
oxide.
The cathode may be formed over a substrate and the anode formed
over the at least one layer of organic material. Alternatively, the
anode may be formed over a substrate and the cathode is formed over
the at least one layer of organic material.
The first layer of inorganic oxide may be sputter deposited,
preferably by DC magnetron or RF sputtering, or evaporated
preferably by resistive or electron-beam thermal evaporation. The
second layer of conductive material may be sputter deposited,
preferably by DC magnetron or RF sputtering, or evaporated
preferably by resistive or electron-beam thermal evaporation.
The ratio of thicknesses of the first layer of inorganic oxide to
the second layer of conductive material is preferably at least
15:1.
The structure of FIG. 5 provides an organic light-emitting device
(OLED) in which the semi-transparent inorganic oxide anode, for
example indium-tin-oxide (ITO), tin oxide, or zinc oxide is coated
with a thin semi-transparent layer of a conductive material with a
work function of at least 4.7 eV prior to the deposition of the
first organic layer of the OLED stack. The thickness of said thin
layer is at most 10 nm but preferably 3-7 nm. Said thin layer can
be Ag, As, Au, C, Co, Ge, Ni, Os, Pd, Pt, Re, Ru, Se, Te or alloys
or inter-metallic compounds containing these elements.
Alternatively said thin layer can be a doped semiconductor such as
p-type doped ZnS or ZnSe. Said thin High work function layer is
covered with at least one organic electroluminescent layer,
preferably a conjugated polymer, and the structure is completed
with a cathode as top electrode.
In a preferred embodiment the think semi-transparent conductive
high work function layer is a layer of carbon between 3 and 7 nm
thick.
In another preferred embodiment the organic electroluminescent film
is a soluble conjugated polymer such as an alkoxy-derivative of
poly(p-phenylene vinylene).
In another embodiment there is provided an OLED which is built up
from a substrate with a cathode as a first electrode, which is then
covered with at least one electroluminescent layer, preferably a
conjugated polymer, and in which the top organic layer is covered
with a thin semi-transparent high work function layer according to
the first aspect of the invention prior to the deposition of a
thicker semi-transparent conductive oxide top anode layer.
The structure of FIG. 5 may be fabricated by a method comprising
the steps of:
forming an anode for the device over a substrate, which step
comprises forming a first light-transmissive layer of an inorganic
oxide over a substrate and forming a second light-transmissive
layer of a conductive material having a high work function over the
first layer of inorganic oxide, the second layer of conductive
material being substantially thinner than the first layer of
inorganic oxide;
forming at least one layer of a light-transmissive organic material
over the anode; and
forming a cathode for the device over the at least one layer of
organic material.
An alternative, "inverse", structure may be fabricated by a method
comprising the steps of:
forming a cathode for the device over a substrate;
forming at least one layer of a light-transmissive organic material
over the anode; and
forming an anode for the device over the at least one layer of
organic material, which step comprises forming a second
light-transmissive layer of a conductive material having a high
work function over the at least one layer of organic material and
forming a first light-transmissive layer of an inorganic oxide over
the second layer of conductive material, the second layer of
conductive material being substantially thinner than the first
layer of inorganic oxide.
In a method of fabrication for an OLED according to the structure
of FIG. 5, a thin semi-transparent high work function layer is
deposited in between a semi-transparent conductive oxide anode and
the adjacent first organic layer. Said thin semi-transparent high
work function layer is applied by sputter deposition or by
resistive or electron-beam thermal evaporation.
Specifically referring now to FIG. 5, a glass substrate 10 is
covered with a layer of a semi-transparent conductive
indium-tin-oxide (ITO) layer 20, typically about 150nm thick with a
sheet resistance of typically .ltoreq.30 Ohms/square. The substrate
may alternatively comprise a plastics material. Said ITO layer 20
is covered with a 6 nm thick layer 30 of electron-beam evaporated
carbon of 99.997% purity. Said layer 30 is then covered with a ca.
100 nm thick layer 40 of poly(2-methoxy-5-(2'-ethyl-hexyloxy)
1,4-phenylene vinylene), abbreviated as MEH-PPV, which is spun onto
layer 30 from a xylene solution. Said MEH-PPV layer is then covered
with a cathode composed of a first layer 50 of ca. 50 nm of Ca
capped with a second protective layer 60 of ca. 200 nm of Al. This
OLED device has improved device performance and in particular
improved operating stability compared to a device without the
carbon layer between the ITO and the MEH-PPV.
Thus there has been described a device structure and process of
fabrication for an OLED with an efficient anode structure for
injecting positive charge carriers which uses a semi-transparent
thin conductive oxide anode to achieve transparency and
conductivity but which avoids direct contact of said conductive
oxide anode with adjacent organic layers which may be degraded in
immediate contact with said oxide anode.
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